During the years of white ceramics, mathematics, and water clocks, the Abbasid dynasty was fairly stable. But no dynasty is ever easily stable; the Abbasid ruler had to move east to Samarra once, as power see-sawed. The Central Asia territory was too large to rule centrally; it kept breaking off at the edges into separatist sultanates. Meanwhile there began a new long-term migration wave of Turks, coming from the east in wagons carrying yurts.
The Abbasid caliphs saw the first Turks as a wonderful resource for building up a private army, the way Abd al-Rahman had done. The families could be offered a bit of land on condition that most of the men join the army; or they could be hired directly; or they could be taken as slaves. The Arabic word for a person owned as a possession has come to us as Mamluk.
Mamluk soldiers were drawn from both western and eastern margins of the empire. Best results came from taking boys who were between 9 and 11, then putting them into Spartan training conditions. The boys adopted the new Arabic identity fairly easily because their training units had high morale. Most of them had already faced a life of hard work and poverty; now they had plenty to eat and could earn rank and rewards.
Mamluk armies became the backbone of the Muslim empire, even until modern times. During the Abbasid period, they were Slavs (often blue-eyed and blond) and Turks (black-haired, with slanted Asian eyes). With a tax-supported Mamluk army, the Abbasid dynasty could resist most of the rebellions.
There were many Turkic ethnic groups in the region at this time. The Pechenegs and Bulgars had been living around Bulgaria, Ukraine and Anatolia since the 7th century. A large group called the Oghuz had settled between the Caspian and Aral Seas, roughly Kazakhstan. Nearby was the Kimak Khanate of the Kipchaks, and roughly in modern Ukraine was the Jewish-convert Khazar Khanate. There were also Uyghurs along the Silk Road. Most of these people shared enough vocabulary to communicate basic words.
A group of Oghuz Turks rebelled, and after a few battles, they left Oghuz territory and started moving westward by stages. Their leader, Seljuk Bey, converted to Islam in the late 900s. His followers began using Arabic Muslim names, so they become hard for us to spot in a cursory glance at history. We see Mahmuds and Ahmeds and assume Arabic ancestry.
It was the beginning of a long, slow, but unstoppable cultural shift within Islam. At the start, the Seljuk Turks were starkly different from their Persian neighbors. However, they learned Persian and borrowed many of its words, and they adapted to its culture. Some of them settled where they were, while others kept the nomadic life and kept drifting.
The first real trouble began in the far east, when a Turkish Mamluk slave took over the territory of modern Pakistan, Afghanistan, and Iran. This coup didn’t directly challenge Baghdad, because the Khorasan region had broken away from its rule already. However, it showed the other Mamluks what was possible. All the Mamluk general Alp Tigin had to do was walk away from his command structure with his troops, take his own city (Ghazna), and begin ruling. Within two generations, the Ghaznavids were Persian-speaking Muslim Sultans who ruled a vast area for about three centuries. Any Mamluk could do it.
By the year 1003, when the mathematical Pope Sylvester II died, Baghdad’s eastern provinces were increasingly Turkish, with little interest in imperial obedience. The Round City itself had Seljuk nomads camped not that many miles away. They weren’t hostile invaders, but they definitely put pressure on the Abbasid status quo. And they thought like nomads.